In general, a gallium nitride based semiconductor has been widely used for ultraviolet, a blue/green light emitting diode, or a laser diode, as a light source of a full color display, a traffic light, a general lighting, and an optical communication device. Particularly, an indium gallium nitride (InGaN) compound semiconductor has increasingly been highlighted due to a narrow band gap thereof.
A light emitting device using the gallium nitride based compound semiconductor has been utilized in various applications such as a large scale full color flat panel display device, a backlight source, a traffic signal, an indoor lighting, a high density light source, a high resolution output system, optical communication, and the like. GaN has a band gap energy of about 3.42 eV, which corresponds to optical energy having a wavelength of about 365 nm. Thus, the light emitting device that uses GaN or InGaN as a well layer, has generally been used to radiate ultraviolet or blue light having a wavelength which is not less than 365 nm. Meanwhile, in order to provide the light emitting diode that radiates ultraviolet having a wavelength which is not more than 365 nm, there is a need to increase a band gap of the well layer, and as a result, the well layer with aluminum (Al) added to GaN or InGaN is used (see Korean Patent Laid Open Publication No. 10-2012-0129449).
Further, a barrier layer or a contact layer includes a higher Al content than an AlGaN or AlInGaN well layer so as to have a wider band gap than the well layer However, as the Al content increases, the AlGaN or AlInGaN layer should be grown at a higher temperature and lower pressure. That is, a growth condition thereof becomes strict, and as a result, it is difficult to form an epi-layer having good crystal quality. Further, as the Al content increases, crystal defects such as cracks or threading dislocations caused by stress are prone to occur in the epi-layer, and as a result, it is difficult to form the epi-layer having good crystal quality.